Organisms that fail to respond appropriately to seasonal change will face severe consequences related to survival and reproduction. For humans, seasonal variation in the amount of light causes Seasonal Affective Disorder (SAD), a recurrent subtype of depression. Although the molecular mechanisms underlying SAD remain elusive, it is known that photoperiodic mechanisms play a significant role in many cases of SAD. Many organisms, including humans, have evolved mechanisms to sense changes in day length (=photoperiod) and integrate seasonal change information into their development. The long-term goal of our research program is to elucidate the molecular mechanisms by which organisms measure changes in day length and adjust their behaviors and development accordingly. Although the molecular mechanisms of photoperiodism have not yet been well described in many organisms, recent advances in the study of the model plant Arabidopsis have increased our molecular understanding of photoperiodic time measurement and have influenced the study of other plant and animal species. In Arabidopsis, the core time-measurement mechanism is circadian-regulated transcription of the floral activator CONSTANS (CO) gene and light-regulated CO protein stability and activity. In this proposal, further characterization o this core mechanism will be done through biochemical, genetic, and genomic approaches. In Aim 1, newly identified transcriptional regulators of CO will be characterized. The findings will elucidate further how the circadian clock plays a role in regulating seasonal response by orchestrating different classes of transcription factors. In Aim 2, time-dependent chromatin modification of CO transcriptional regulation - a new mechanistic layer of understanding - will be studied. Our previous results indicated the involvement of the FKF1 blue-light photoreceptor in posttranslational regulation of CO protein. In Aim 3, the molecular function of FKF1-associating proteins on CO protein stability regulation will be examined. With these three Aims, the mechanisms by which plants monitor daily and seasonal differences through the circadian clock will be analyzed. The findings will impact plant research and our broader understanding of photoperiodism and circadian clocks in mammals and other systems. The types of transcriptional and posttranslational mechanisms present in Arabidopsis are likely conserved among all eukaryotes, thus these findings will contribute to our understanding of fundamental transcriptional regulation. These findings also have the potential to provide clues to the mechanisms involved in SAD. Finally, elucidating the photoperiodic flowering mechanism is important for understanding a major plant reproduction mechanism that is directly applicable to improvements in crop yield, an important contributor to human health especially in developing countries.